Differential topologies offer key advantages for integrated devices such as mixers, voltage-controlled oscillators and amplifiers, providing better speed, frequency response and noise performance. Moreover, “balanced” antenna topologies are useful for wideband, efficient, and well-isolated transceiver front- and back-ends. As such, differential-mode circuit topologies are widely utilized for the design of high performance microwave monolithically integrated circuits (MMICs) spanning RF to lower-mmW bands. Nevertheless, for higher-mmW and sub-mmW bands (>100 GHz), design and characterization of differential-mode on-chip devices and circuits have long been a technical challenge. Therefore, despite their advantages, research and development of differential terahertz monolithically integrated circuits (TMICs) are impeded by the lack of measurement and characterization tools.
For differential-mode on-wafer device characterization, dual-tip coplanar, coaxial probes are interfaced with either four-port, dual-source VNAs (vector network analyzer) or two-port VNAs are used in conjunction with hybrids/couplers for a pure-mode VNA (PMVNA). While dual-source, four-port VNAs are limited by the deteriorating phase noise at higher frequencies, pure-mode VNA concepts are limited by the availability of components and interconnect elements beyond 110 GHz.
As an alternative, balun-integrated probes were introduced for pure differential-mode measurements. In such micro-machined probe architectures, a Marchand-type balun is fabricated onto the dual, coplanar contact probe tip membrane. The balun converts the conventional test signal injected by the VNA into an on-wafer, pure differential-mode excitation, while suppressing any common mode signals emerging from the discontinuities in the fixture or the on-chip device under test. Evidently, this approach involves multilayered lithography to fabricate the balun on the probe membrane, leading to increased manufacturing and maintenance costs. Furthermore, most recent prototypes reported in the literature cover only up to 110 GHz, leaving much of the sub-mmW spectrum out of reach.
Alternatively, a balun can be fabricated on the same wafer as the device under test (DUT) and conventional, single-tip probes can be used to characterize the response. Nevertheless, fabrication of such on-wafer baluns also require a fairly complex process, adding to the overall cost and introducing fabrication uncertainties such as yield. Moreover, such on-wafer baluns are needed for each and every device under test, leading to prohibitive costs.